What’s the News: Researchers have mapped out the detailed geological history of a 300-square-mile chunk of New Zealand, from 2.5 million years ago to the present day. The study showed how glaciers carved out the area’s distinctive valleys using a little-known technique called thermochronometry, which involves shooting proton beams onto rocks and making note of what happens—along with some impressive analytical skills.

How the Heck: Thermochronometry, as the name implies, is a way to measure both temperature and time. The general principle is that the deeper something is below the Earth’s surface, the warmer it is; thus, tracking a rock’s temperature over millions of years reveals how deep the rock was as it cooled.

This study used a particular version of the method called helium-4/helium-3 thermochronometry, first developed by a member of the research team in 2005. This technique lets you track the time and temperature for apatite, a mineral found in rocks like granite that solidify from liquid magma far beneath the surface. Two chemical elements in apatite—uranium and thorium—are radioactive. As they decay over millions of years, they produce helium-4, the common version, or “isotope,” of that element.

That’s where time and temperature come in: The rate at which helium-4 leaks out into nearby rock slows as the apatite cools. The researchers shined a proton beam onto apatite-laden rocks, which creates the lighter helium isotope, helium-3. This reaction sends both types of helium hissing out of the rock as a gas, which the researchers can then measure for helium-4 levels. Once they know that, they can work backward to figure out how quickly the rock cooled—and thus, where it was over the past 2.5 million years or so, providing a picture of how the Earth’s surface has changed.

What’s the Context: Knowing how deep a rock was, and when it was there, is a big deal, given that Earth’s outermost layer is constantly changing due to everything from earthquakes to agriculture. As the Thermochronometry Research Lab at University College London puts it, the technique can help researchers “understand how tectonic and erosional processes have shaped the Earth’s surface … and influenced global and regional climate change.”

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